It is well known that Escherichia coli executes chemotactic motion in response to chemical cues by modulating the flagellar motor bias alone. However, previous studies have reported the possibility of variation in run speed in the presence of attractants although it is unclear whether bacteria can deliberately modulate their swimming speeds in response to environmental cues or if the motor speeds are hardwired. By studying the detailed motion of cells in a uniform concentration of glucose and its non-metabolizable analogue, we show that changing concentrations may be accompanied by variation in the swimming speed. For a fixed run duration, cells exposed to the attractants achieved a higher peak-swimming speed after a tumble compared with that in plain motility buffer. Our experiments using the mutant strain lacking the Trg sensor show no change in swimming speed with varying concentrations of the non-metabolizable analogue, suggesting that sensing may play a role in the observed variation of swimming speed.

Studies on chemotaxis of Escherichia coli have shown that modulation of tumble frequency causes a net drift up the gradient of attractants. Recently, it has been demonstrated that the bacteria is also capable of varying its runs speed in uniform concentration of attractant. In this study, we investigate the role of swimming speed on the chemotactic migration of bacteria. To this end, cells are exposed to gradients of a non-metabolizable analogue of glucose which are sensed via the Trg sensor. When exposed to a gradient, the cells modulate their tumble duration, which is accompanied with variation in swimming speed leading to drift velocities that are much higher than those achieved through the modulation of the tumble duration alone. We use an existing intra-cellular model developed for the Tar receptor and incorporate the variation of the swimming speed along with modulation of tumble frequency to predict drift velocities close to the measured values. The main implication of our study is that E. coli not only modulates the tumble frequency, but may also vary the swimming speed to affect chemotaxis and thereby efficiently sample its nutritionally rich environment.

No abstract

No abstract

developed a dual-chain biosensor based on intermolecular FRET that enables the spatio-temporal dynamics of Rap1 activity to be measured in individual living cells. The FRET generated by this dual-chain sensor is a measure of the binding interaction between cerulean-tagged Rap1 and an affinity reagent fused to yPet. To facilitate expression of the sensor and to reduce cell-to-cell heterogeneity in the stoichiometry of components we have engineered a tandem viral-2A (tv-2A) mediated expression vector that achieves over 99% separation of proteins when both chains of the biosensor are transfected as a single gene. In living cells this novel Rap1 biosensor showed a > 10-fold increase in FRET upon activation, and is sufficiently bright to image Rap1 activity at concentrations that do not interfere with normal cell motility. The intermolecular design of this biosensor enables native regulation of Rap1 localization by CAAX-box modification and is well suited to fluorescence lifetime imaging microscopy (FLIM), a technique that greatly enhances quantitative accuracy and multiplexing potential.